Method of making a dual-cavity pressure sensor die

ABSTRACT

A pressure sensor die especially suitable for high-temperature, high-pressure operating environment and delivering accurate and reliable pressure measurement at low cost. A single crystalline silicon includes a cap, a substrate and a base connected together. A recess formed on the cap creates an upper sealed cavity with the substrate. A silicon oxide layer is formed between the substrate and the cap. A recess formed on the base creates a lower sealed cavity with the substrate. The upper sealed cavity and the lower sealed cavity overlap in their projections. The substrate includes at least two sets of piezoresistive sensing elements located within the overlapping projections, perpendicular to each other, and oriented in different crystallographic directions.

The present invention is related to a sensor. In particular, the presentinvention relates to a pressure sensor for downhole pressuremeasurements.

BACKGROUND OF THE INVENTION

Downhole pressure measurements are essential when drilling forhydrocarbon recovery. During the drilling process, geological pressuredata are collected to tailor drilling parameters and the construction ofthe well. After the well is drilled and production starts, pressure iscontinuously monitored for reservoir management. Accurate measurement ofpressure is therefore the key to optimize recovery and reduce riskthroughout the entire life of a hydrocarbon well. Thus, we need anaccurate and cost-effective pressure sensor for downhole measurements.

Pressure sensors usable in hydrocarbon wells must be able to withstandharsh conditions and remain accurate, stable and reliable for weeksduring a measurement period. In particular, such sensors must be able towithstand temperature ranging from −50° C. to 250° C. and pressure up to200 MPa (around 2000 atmospheres) while maintaining an accuracy ofbetter than 0.1%, and desirably 0.01%, of the full-scale pressure.

Two types of pressure sensors are commonly used for downholeapplications. The first type is the resonant quartz pressure sensor. InU.S. Pat. No. 3,617,780, one example of resonant quartz pressure sensoris described wherein a crystalline quartz cylinder closed at both endsis immersed in a fluid which communicates with the external pressure tobe measured via an isolation diaphragm or a bellow. A crystalline quartzplate spans across the vacuum sealed cavity inside the cylinder. Theplate resonance is excited and detected via the piezoelectric effect.The plate resonant frequency, which varies with the hydrostatic pressureon the cylinder wall, is a measure of the external pressure. Constructedalmost entirely out of crystalline quartz and being a mature technology,resonant quartz pressure sensors have achieved the highest benchmark foraccuracy, stability and reliability for downhole pressure measurementsto date. However, they tend to be very expensive.

The second type of downhole pressure sensors is based on sapphire. InU.S. Pat. No. 5,024,098, a sapphire pressure sensor is described whereina sapphire cell is immersed in a fluid which communicates with theexternal pressure to be measured via an isolation diaphragm. The celldeforms under pressure and the resulting strains are measured by straingauge elements disposed on a planar surface of the sapphire cell. Whilereliable and rugged for downhole applications, sapphire pressure sensorsare in general not as stable and accurate as resonant quartz pressuresensors and they are also quite expensive. In case silicon strain gaugeelements are employed, accuracy and stability could be affected by theexcessive temperature coefficient of resistance and the temperaturecoefficient of piezoresistance effect in silicon. On the other hand, ifnon-silicon strain gauge elements, for example, metallic alloys, areused their low gauge factor and therefore low sensitivity can result inthe undesirable amplification of temperature and other measurementerrors. In any case, the mismatch in the thermal expansion coefficientsbetween sapphire and the strain gauge material creates furthertemperature errors.

The majority of sensors in use today are of the micro-electro-mechanicalsystem (MEMS) type. MEMS based sensors are typically realized withsilicon micromachining that originated from integrated circuitfabrication and still shares many of its processing technologies. Inaddition, there are a few unique processes specifically tailored towardthe fabrication of 3-dimensional microstructures. These includedouble-side photolithography, deep reactive ion etching (DRIE), andwafer bonding to name a few. Silicon has superb mechanical propertiescompared with quartz and sapphire, for example, high hardness, highmodulus of elasticity, high ultimate strength, and is perfectly elasticup to the fracture point. Moreover, single crystalline silicon is highlypiezoresistive, which is therefore effective in converting changes inmechanical strain into changes in electrical resistance. Furthermore,precision microstructures are much easier to fabricate in silicon thanin quartz or sapphire. With demonstrated advantages that include lowcost, small size, high accuracy, high reliability, and high stability,silicon MEMS piezoresistive pressure sensors have become the dominanttype of pressure sensors in use for automotive, medical, industrial andconsumer electronics applications.

Despite their huge success, MEMS pressure sensors have not been widelyadopted for downhole applications. There are a few problems that must beovercome. In particular, an improved mechanical design over theconventional diaphragm-type silicon pressure sensors is required tohandle the very high pressure. This is because in a conventionaldiaphragm-type silicon pressure sensor die, the silicon thin diaphragmserves the purpose of amplifying pressure into stress. In order tomeasure high pressure up to 200 MPa, the lateral dimensions andthickness of the diaphragm must be respectively narrowed down andthickened accordingly. However, if the lateral dimensions of thediaphragm are made too narrow, there will not be enough room on thediaphragm to place the piezoresistive sensing elements. Else if thediaphragm is thickened substantially, it will lead to non-idealdeformation of the entire pressure sensor die. Furthermore, thepiezoresistive sensing elements in conventional MEMS pressure sensorsare located on the pressure sensor die surface where they are morevulnerable to external environmental influence. Besides, there needs tobe a better means to overcome the various temperature coefficients andinstabilities so as to improve the measurement accuracy at hightemperature. Accordingly, a need presently exists for an improvedsilicon pressure sensor that is highly accurate, cost effective, andsuitable for operating in a high temperature, high pressure downholeenvironment.

SUMMARY OF THE INVENTION

The objective of the present invention is to overcome currenttechnological shortcomings so as to provide a pressure sensor that ishighly accurate, having a wide pressure range, less affected by theenvironment, and capable of operating in a high temperature, highpressure downhole environment.

A pressure sensor die comprising:

a cap, a substrate and a base bonded together; said pressure sensor dieis constructed of single crystalline silicon;

wherein a recess is formed on said cap, said recess formed on said capbonds with said substrate and forms an upper sealed cavity; anotherrecess is formed on said base, said recess formed on said base bondswith said substrate and forms a lower sealed cavity; said substrate ispositioned between said upper and lower sealed cavities and partitionssaid upper and lower sealed cavities;

a silicon oxide layer is formed between said substrate and said cap;

said upper sealed cavity and said lower sealed cavity overlap in theirprojections;

said substrate further comprises at least two sets of piezoresistivesensing elements; said piezoresistive sensing elements are locatedwithin the projections of said upper sealed cavity and said lower sealedcavity; said two sets of piezoresistive sensing elements areperpendicular to each other, with each set of piezoresistive sensingelements oriented in a different crystallographic direction.

The pressure sensor die in the present invention also comprises thefollowing additional features:

Said upper sealed cavity and said lower sealed cavity are vacuum sealedcavities.

Metal contacts are provided at the terminals of said piezoresistivesensing element.

Said piezoresistive sensing element comprises a plurality of connectedU-shaped segments.

Said piezoresistive sensing elements are electrically connected in aWheatstone bridge configuration.

Said substrate is formed on a {110} crystallographic plane of p-typesilicon; said piezoresistive sensing elements are formed on n-type dopedregions of said substrate; one set of said piezoresistive sensingelements is oriented along a <100> crystallographic direction, and theother set of said piezoresistive sensing elements is oriented along a<110> crystallographic direction.

Said substrate is formed on a {110} crystallographic plane of n-typesilicon; said piezoresistive sensing elements are formed on p-type dopedregions of said substrate; one set of said piezoresistive sensingelements is oriented along a <100> crystallographic direction, and theother set of said piezoresistive sensing elements is oriented along a<110> crystallographic direction.

Said substrate of said pressure sensor die uses a silicon-on-insulatorconstruction comprising:

a handle layer, a device layer, and a buried silicon oxide layer formedbetween said handle layer and device layer;

said piezoresistive sensing elements are formed on said device layer.

A silicon oxide insulating layer is formed on the top, the bottom andalong the sides of said piezoresistive sensing element.

Said device layer is formed on a {110} crystallographic plane of p-typesilicon; said piezoresistive sensing elements are formed on said p-typesilicon of said device layer; one set of said piezoresistive sensingelements is oriented along a <100> crystallographic direction, and theother set of said piezoresistive sensing elements is oriented along a<110> crystallographic direction.

Said device layer is formed on a {110} crystallographic plane of n-typesilicon; said piezoresistive sensing elements are formed on said n-typesilicon of said device layer; one set of said piezoresistive sensingelements is oriented along a <100> crystallographic direction, and theother set of said piezoresistive sensing elements is oriented along a<110> crystallographic direction.

A pressure sensor comprising:

a chamber, an electrically insulating fluid that fills said chamber, andsaid pressure sensor die provided within said chamber;

said chamber is an enclosure within a metal housing;

said pressure sensor die is immersed in said electrically insulatingfluid.

A metal diaphragm is further provided in said pressure sensor; saidmetal diaphragm is connected to said chamber; said metal diaphragm sealssaid electrically insulating fluid and said pressure sensor die in saidchamber; and external pressure to be measured is transmitted from saidmetal diaphragm to said pressure sensor die.

The first fabrication process for the first embodiment of said pressuresensor die;

the starting material for said substrate is a single crystalline siliconwafer;

said fabrication process comprises the following steps:

-   -   Step 1, grow or deposit a silicon oxide layer on the top surface        of a substrate silicon wafer;    -   Step 2, using photolithography and ion implantation, dope        selective regions on the top surface of said substrate silicon        wafer, thus forming a plurality of piezoresistive sensing        elements with the opposite dopant type to said substrate silicon        wafer;    -   Step 3, using photolithography and ion implantation, highly dope        selective regions on the top surface of said substrate silicon        wafer, thus forming highly conductive regions with the opposite        dopant type to said substrate silicon wafer;    -   Step 4, using photolithography and ion implantation, highly dope        selective regions on the top surface of said substrate silicon        wafer, thus forming highly conductive regions with the same        dopant type as said substrate silicon wafer; afterward grow or        deposit a silicon oxide layer on the top surface of said        substrate silicon wafer, and activate said implanted dopant        species in said piezoresistive sensing elements, said highly        conductive regions with the opposite dopant type to said        substrate silicon wafer, and said highly conductive regions with        the same dopant type as said substrate silicon wafer;    -   Step 5, using photolithography and etching, etch contact holes        through said silicon oxide layer over said highly conductive        regions with the opposite dopant type to said substrate silicon        wafer, and over said highly conductive regions with the same        dopant type as said substrate silicon wafer; then use metal        deposition to form metal interconnection patterns from said        contact holes;    -   Step 6, bond a cap silicon wafer which has been prefabricated        with recesses to the top surface of said substrate silicon        wafer;    -   Step 7, grind and thin down the bottom side of said substrate        silicon wafer;    -   Step 8, bond a base silicon wafer which has been prefabricated        with recesses to the bottom surface of said substrate silicon        wafer;    -   Step 9, using wafer dicing, cut said bonded cap, substrate, and        base silicon wafers into completed individual pressure sensor        dice.

The second fabrication process for the first embodiment of said pressuresensor die;

the starting material for said substrate is a single crystalline siliconwafer;

said fabrication process comprises the following steps:

-   -   Step 1, fabricate recesses on the top surface of a base silicon        wafer;    -   Step 2; bond said base silicon wafer to the bottom surface of a        substrate silicon wafer; afterward grind and thin down the top        side of said substrate silicon wafer;    -   Step 3, grow or deposit a silicon oxide layer on the top surface        of said substrate silicon wafer;    -   Step 4, using photolithography and ion implantation, dope        selective regions on the top surface of said substrate silicon        wafer, thus forming a plurality of piezoresistive sensing        elements with the opposite dopant type to said substrate silicon        wafer;    -   Step 5, using photolithography and ion implantation, highly dope        selective regions on the top surface of said substrate silicon        wafer, thus forming highly conductive regions with the opposite        dopant type to said substrate silicon wafer;    -   Step 6, using photolithography and ion implantation, highly dope        selective regions on the top surface of said substrate silicon        wafer, thus forming highly conductive regions with the same        dopant type as said substrate silicon wafer; afterward grow or        deposit a silicon oxide layer on the top surface of said        substrate silicon wafer, and activate said implanted dopant        species in said piezoresistive sensing elements, said highly        conductive regions with the opposite dopant type to said        substrate silicon wafer, and said highly conductive regions with        the same dopant type as said substrate silicon wafer;    -   Step 7, using photolithography and etching, etch contact holes        through said silicon oxide layer over said highly conductive        regions with the opposite dopant type to said substrate silicon        wafer, and over said highly conductive regions with the same        dopant type as said substrate silicon wafer; then use metal        deposition to form metal interconnection patterns from said        contact holes;    -   Step 8, bond a cap silicon wafer which has been prefabricated        with recesses to the top surface of said substrate silicon        wafer;    -   Step 9, using wafer dicing, cut said bonded cap, substrate, and        base silicon wafers into completed individual pressure sensor        dice.

The first fabrication process for the second embodiment of said pressuresensor die;

the starting material for said substrate is a silicon-on-insulator wafercomprising a handle layer, a device layer, and a buried silicon oxidelayer formed between said handle layer and device layer;

said fabrication process comprises the following steps:

-   -   Step 1, grow or deposit a silicon oxide layer on the top surface        of said device layer;    -   Step 2, using photolithography and ion implantation, highly dope        selective regions on the top surface of said device layer, thus        forming highly conductive regions with the same dopant type as        said device layer;    -   Step 3, using photolithography and etching, etch trenches        through said device layer reaching said buried silicon oxide        layer, thus forming a plurality of piezoresistive sensing        elements;    -   Step 4, grow or deposit a layer of silicon oxide to fill said        trenches, and activate said implanted dopant species in said        highly conductive regions;    -   Step 5, using photolithography and etching, etch contact holes        through said silicon oxide layer over said highly conductive        regions reaching said highly conductive regions in said device        layer; then use metal deposition to form metal interconnection        patterns from said contact holes;    -   Step 6, bond a silicon cap wafer which has been prefabricated        with recesses to the top surface of said substrate;    -   Step 7, grind and thin down the bottom side of said substrate;    -   Step 8, bond a base silicon wafer which has been prefabricated        with recesses to the bottom surface of said substrate;    -   Step 9, using wafer dicing, cut said bonded cap, substrate, and        base silicon wafer into completed individual pressure sensor        dice.

The second fabrication process for the second embodiment of saidpressure sensor die;

the starting material for said substrate is a silicon-on-insulator wafercomprising a handle layer, a device layer, and a buried silicon oxidelayer formed between said handle layer and device layer;

said fabrication process comprises the following steps:

-   -   Step 1, fabricate recesses on the top surface of a base silicon        wafer;    -   Step 2, grind and thin down the bottom side of said substrate;        afterward bond said base silicon wafer to the bottom surface of        said substrate;    -   Step 3, grow or deposit a silicon oxide layer on the top surface        of said device layer on said bonded substrate and base silicon        wafer;    -   Step 4, using photolithography and ion implantation, highly dope        selective regions on the top surface of said device layer, thus        forming highly conductive regions with the same dopant type as        said device layer;    -   Step 5, using photolithography and etching, etch trenches        through said device layer reaching said buried silicon oxide        layer, thus forming a plurality of piezoresistive sensing        elements;    -   Step 6, grow or deposit a layer of silicon oxide to fill said        trenches, and activate said implanted dopant species in said        highly conductive regions;    -   Step 7, using photolithography and etching, etch contact holes        through said silicon oxide layer over said highly conductive        regions reaching said highly conductive regions in said device        layer; then use metal deposition to form metal interconnection        patterns from said contact holes;    -   Step 8, bond a cap silicon wafer which has been prefabricated        with recesses to the top surface of said substrate;    -   Step 9, using wafer dicing, cut said bonded cap silicon wafer,        said substrate, and the base silicon wafer into completed        individual pressure sensor dice.

The fabrication process for said recesses on said silicon cap wafer andsaid base silicon wafer comprises photolithography and etching.

Said etching method comprises one kind or a combination of dry and wetetching methods; said dry etching method is selected from one or more ofthe following methods: deep reactive ion etching, reactive ion etching,or gaseous xenon difluoride etching for silicon; as well as reactive ionetching, plasma etching, or hydrofluoric acid vapor etching for siliconoxide.

Said wet etching method for silicon comprises one kind or a combinationof the following etchants: potassium hydroxide, tetramethylammoniumhydroxide, or ethylenediamine pyrocatechol.

Said wet etching method for silicon oxide comprises one kind or acombination of the following etchants: hydrofluoric acid or bufferedhydrofluoric acid.

Comparing with the two types of downhole pressure sensors mentioned inthe prior art, the pressure sensor in the present invention has thefollowing advantages. First of all, the manufacturing cost of a siliconpressure sensor is much lower than that for quartz and sapphire pressuresensors. However, conventional diaphragm-type silicon pressure sensorscannot function in a 200 MPa environment. In contrast, the pressuresensor die in the present invention does not utilize a diaphragm elementas in conventional silicon pressure sensors. Instead, the sensor die isacted upon on all of its surfaces (the top, the bottom and the foursides) by the high pressure in the downhole. The external pressure isdirectly converted by means of the dual-cavity structure into internalstresses in the substrate of the pressure sensor die without the needfor mechanical amplification by a silicon diaphragm. This way thepresent invention overcomes the main difficulty in the mechanical designof diaphragm-type silicon pressure sensors for high pressureapplications while retaining the advantages of silicon MEMS pressuresensors.

Secondly, conventional MEMS piezoresistive sensing elements areelectrically insulated by reverse biased PN junctions, the leakagecurrent of which increases exponentially with temperature. As thetemperature rises above 150° C., the insulation property of the PNjunction will fail. On the other hand, in one of the preferredembodiments of the present invention, there is a buried silicon oxidelayer between the piezoresistive sensing elements and the handle layer.There is also silicon oxide layer in between each piezoresistive sensingelement. Moreover, a silicon oxide layer is grown or deposited on top ofthe piezoresistive sensing elements. As a result, each piezoresistivesensing element is completely wrapped around by silicon oxideinsulation. Using this dielectric isolation scheme, the electricalinsulation will operate even at high temperature.

Furthermore, in the present invention, the two perpendicular sets ofpiezoresistive sensing elements on the pressure sensor die havedifferent pressure responses, hence providing a differential change inelectrical resistance. Then by connecting all four piezoresistivesensing elements in a Wheatstone bridge configuration, temperature andother common mode errors are significantly reduced, thereby increasingthe accuracy of the present pressure sensor.

Additionally, the internal cavity formed between the substrate and thecap is preferably sealed in vacuum. The critical portions of all thepiezoresistive sensing elements are located inside this vacuum sealedcavity; as a result, these critical portions of the piezoresistivesensing elements are least susceptible to external interferences, suchas local temperature fluctuations, and foreign contaminations, such asdust. The reliability of the present pressure sensor is further improvedas a result. Lastly, the entire pressure sensor die primarily uses asilicon construction which not only avoids the problems caused by themismatch between dissimilar materials, but also enables the use of MEMSfabrication technologies with much lower manufacturing cost than thatfor the quartz and sapphire pressure sensors in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the pressure sensor die in the firstembodiment of this invention.

FIG. 2 is a perspective view of the pressure sensor die of FIG. 1further with the cap and base detached and further with the top siliconoxide layer removed revealing various features on the substrate in thefirst embodiment of this invention.

FIG. 3 is a plan view of the pressure sensor die in the first embodimentof this invention.

FIG. 4 is a perspective view of the pressure sensor die in the secondembodiment of this invention.

FIG. 5 is a perspective view of the pressure sensor die of FIG. 4further with the cap and base detached and further with the top siliconoxide layer removed revealing various features on the device layer inthe second embodiment of this invention.

FIG. 6 is a cutaway view of the pressure sensor die of FIG. 5 along lineAA′.

FIG. 7 is a plan view of the pressure sensor die in the secondembodiment of this invention.

FIG. 8 is a circuit diagram of the piezoresistive sensing elementsconnected in a Wheatstone bridge configuration.

FIG. 9 is a diagrammatic view of the pressure sensor.

FIG. 10A is the variation of the piezoresistive coefficients π11+π12versus crystallographic orientation on a {110} crystallographic plane ofp-type silicon.

FIG. 10B is the variation of the piezoresistive coefficients π11+π12versus crystallographic orientation on a {110} crystallographic plane ofn-type silicon.

FIG. 11 is a cross-sectional view illustrating step 1 and step 2 of thefirst fabrication process in the first embodiment of the pressure sensordie.

FIG. 12 is a cross-sectional view illustrating step 3 and step 4 of thefirst fabrication process in the first embodiment of the pressure sensordie.

FIG. 13 is a cross-sectional view illustrating step 5 and step 6 of thefirst fabrication process in the first embodiment of the pressure sensordie.

FIG. 14 is a cross-sectional view illustrating step 7 and step 8 of thefirst fabrication process in the first embodiment of the pressure sensordie.

FIG. 15 is a cross-sectional view illustrating step 9 of the firstfabrication process in the first embodiment of the pressure sensor die.

FIG. 16 is a cross-sectional view illustrating step 1 and step 2 of thesecond fabrication process in the first embodiment of the pressuresensor die.

FIG. 17 is a cross-sectional view illustrating step 3 and step 4 of thesecond fabrication process in the first embodiment of the pressuresensor die.

FIG. 18 is a cross-sectional view illustrating step 5 and step 6 of thesecond fabrication process in the first embodiment of the pressuresensor die.

FIG. 19 is a cross-sectional view illustrating step 7 and step 8 of thesecond fabrication process in the first embodiment of the pressuresensor die.

FIG. 20 is a cross-sectional view illustrating step 9 of the secondfabrication process in the first embodiment of the pressure sensor die.

FIG. 21 is a cross-sectional view illustrating step 1 and step 2 of thefirst fabrication process in the second embodiment of the pressuresensor die.

FIG. 22 is a cross-sectional view illustrating step 3 and step 4 of thefirst fabrication process in the second embodiment of the pressuresensor die.

FIG. 23 is a cross-sectional view illustrating step 5 and step 6 of thefirst fabrication process in the second embodiment of the pressuresensor die.

FIG. 24 is a cross-sectional view illustrating step 7 and step 8 of thefirst fabrication process in the second embodiment of the pressuresensor die.

FIG. 25 is a cross-sectional view illustrating step 9 of the firstfabrication process in the second embodiment of the pressure sensor die.

FIG. 26 is a cross-sectional view illustrating step 1 and step 2 of thesecond fabrication process in the second embodiment of the pressuresensor die.

FIG. 27 is a cross-sectional view illustrating step 3 and step 4 of thesecond fabrication process in the second embodiment of the pressuresensor die.

FIG. 28 is a cross-sectional view illustrating step 5 and step 6 of thesecond fabrication process in the second embodiment of the pressuresensor die.

FIG. 29 is a cross-sectional view illustrating step 7 and step 8 of thesecond fabrication process in the second embodiment of the pressuresensor die.

FIG. 30 is a cross-sectional view illustrating step 9 of the secondfabrication process in the second embodiment of the pressure sensor die.

DETAILED DESCRIPTION

The illustrative embodiments of the present invention will be describedin detail with reference to the accompanying drawings. Please note thatthe scope of the present invention is not limited to these preciseembodiments described. Various changes or modifications may be effectedtherein by one skilled in the art without departing from the scope orspirit of the invention.

With reference to FIGS. 1-3, a pressure sensor die is shown according tothe first embodiment of the present invention. The pressure sensor dieis primarily constructed of single crystalline silicon and comprises abase 7, a substrate 6 and a cap 3 connected together. Using a siliconconstruction helps reduce the measurement errors caused by the mismatchin thermal expansion coefficients between dissimilar materials. A recess5 is formed on the cap 3, which forms an upper sealed cavity afterbonding with the substrate 6. Another recess 5 is formed on the base 7,which forms a lower sealed cavity after bonding with the substrate 6.The upper sealed cavity and the lower sealed cavity overlap in theirprojections. In the plane view shown in FIG. 3, the bonded plane betweenthe cap and the substrate is outlined by the dashed lines. The substratefurther comprises a plurality of piezoresistive sensing elements 23. Thecritical portions of the piezoresistive sensing elements 23 are locatedinside the upper sealed cavity, and within the boundaries defined by theprojections of the upper sealed cavity and the lower sealed cavity. Theupper sealed cavity and lower sealed cavity are partitioned by thesubstrate 6. The two cavities do not communicate with each other.Preferably, the upper sealed cavity and lower sealed cavity are vacuumsealed cavities; this further reduces the undesirable effects of foreigncontaminants and local temperature fluctuations on the piezoresistivesensing elements 23. A silicon oxide layer 4 is formed between substrate6 and cap 3. Metal contacts 8 are provided on top of silicon oxide layer4. These metal contacts 8 are respectively connected to the terminals ofpiezoresistive sensing elements 23. External electrical circuits andcomponents are only connected to metal contacts 8; thus it furtherreduces the effects of undesirable interferences on piezoresistivesensing elements 23.

With reference to FIG. 3, in this first embodiment of the presentinvention, four identical piezoresistive sensing elements 23 areprovided on the substrate 6. These are R1 to R4 in which R1, R3 and R2,R4 are perpendicular to each other. Each piezoresistive sensing elementemploys a basic U-shaped design. Preferably, the piezoresistive sensingelement 23 comprises a few U-shaped segments connected together to forma serpentine structure. In the first embodiment, piezoresistive sensingelements 23 are diffused resistors formed by doping selective regions onsubstrate 6. They are electrically insulated from each other viareversed biased PN junctions. The dopant type for piezoresistive sensingelements 23 is according to whether substrate 6 is p-type or n-type. Forp-type substrate 6, an n-type dopant is used. For n-type substrate 6, ap-type dopant is used instead. Preferably, each piezoresistive sensingelement 23 is further formed with type-A highly doped regions 9, thepurpose of which is to increase the doping level in selective portionsof piezoresistive sensing element 23. Therefore, the dopant type for thetype-A highly doped regions 9 must be the same as the dopant type in theoriginal non-highly doped portions of piezoresistive sensing element 23.Since the sheet resistance in the non-highly-doped portion isapproximately 100 Ω/square whereas the sheet resistance in thehighly-doped portion is only 15 Ω/square, having type-A highly dopedregions 9 in each piezoresistive sensing element 23 locally reduces theelectrical resistance, thus forming highly conductive regions. As aresult, the total electrical resistance in each piezoresistive sensingelement 23 primarily comes from a few remaining longitudinal segmentsthat are non-highly doped. In actual pressure measurements, it is thesenon-highly doped longitudinal segments, which are housed inside theupper sealed cavity and oriented in the same direction, that produce thelargest electrical resistance change. Reducing the electrical resistanceof piezoresistive sensing elements 23 in selective regions using type-Ahighly doped regions 9 therefore increases the overall percentage changein the electrical resistance of piezoresistive sensing element 23; andhence improves the measurement accuracy of the pressure sensor.Moreover, through the connections between the metal contacts 8 and thetype-A highly doped regions 9, good electrical contacts between metalcontacts 8 and piezoresistive sensing elements 23 can be assured.Additionally, type-B highly doped regions 10 and their associated metalcontacts 8 are further provided, the purpose of which is for makingelectrical connections between external electrical circuit and theremaining regions of substrate 6 that are not a part of thepiezoresistive sensing elements 23. These electrical connections providethe necessary reverse bias for the PN junctions. The dopant type forthese type-B highly doped regions must therefore be the same as thesubstrate 6 dopant type to ensure that the metal contacts 8 makes goodelectrical contacts with the substrate 6.

Since the four piezoresistive sensing elements R1 to R4 are identical,when the pressure sensor die is not subjected to an external pressure,the electrical resistance in R1 to R4 should be the same in theory.Preferably, the two perpendicular sets of piezoresistive sensingelements, R1, R3 and R2, R4, are oriented along differentcrystallographic directions such that when the pressure sensor die isuniformed compressed and free to deform under the external pressure, thedifference in the piezoresistance effect between the two sets ofpiezoresistive sensing elements, R1, R3 and R2, R4, is maximized, thusresulting in unequal electrical resistance changes. Stress, however, isnot the only factor that affects the electrical resistance values inpiezoresistive sensing elements 23. Other factors, such as thetemperature in the environment, will change the electrical resistancevalues as well. For this, preferably and with reference to FIG. 8,piezoresistive sensing elements R1 to R4 are electrically connected in aWheatstone bridge configuration. A constant current source 24 suppliesthe electric current going through the Wheatstone bridge. The externalpressure can be calculated by measuring the voltage difference betweenpoints V+ and V−. When the pressure sensor die is not subjected toexternal pressure, the electrical resistances in piezoresistive sensingelements R1 to R4 are almost identical; the voltage between points V+and V− is then close to zero. On the other hand, when an externalpressure induces unequal electrical resistance changes between R1, R3and R2, R4, a voltage develops between points V+ and V−. A majoradvantage of the Wheatstone bridge configuration is the reduction ofcommon-mode errors. For example, as the temperature changes, theelectrical resistances of all four piezoresistive sensing elementschange by about the same amount. As a result, when there is no externalpressure, the voltage between points V+ and V− remains close to zero. Inthe present invention, the Wheatstone bridge can be powered by aconstant voltage source or a constant current source, but a constantcurrent source is preferred because the negative temperature coefficientof silicon piezoresistance effect is partially offset by the positivetemperature coefficient of resistance, resulting in an overall reducedscale factor error over temperature. Furthermore, the bridge voltage,represented by Vb which can be measured, contains temperatureinformation which is useful for further temperature error correction.

With reference to FIGS. 1 and 3, the preferable size of the presentpressure sensor die is approximately 1.6 mm in length, 1.6 mm in widthand 1.2 mm in thickness; the respective thicknesses of the cap,substrate and base are approximately 0.5, 0.2 and 0.5 mm. The uppersealed cavity and the lower sealed cavity each measures approximately0.4 mm in length, 0.4 mm in width and 0.2 mm in height. Inmanufacturing, an 8 inch silicon wafer can contain thousands to over10,000 gross pressure sensor dice, thus resulting in a significantreduction in die cost. However, it should be noted that the foregoingdimensions of the preferred embodiment are for illustrative purposesonly. The present invention is not limited to this embodiment and alldimensions can be tailored for a particular design.

In the first embodiment, the piezoresistive sensing elements 23 areelectrically insulated by reverse biased PN junctions, the leakagecurrent of which increases exponentially with temperature. As thetemperature rises above 150° C., the insulating property of the reversebiased PN junction will fail. Therefore the first embodiment is onlysuitable for applications in which the temperature is below 150° C.

With reference to FIGS. 4-7, a pressure sensor die is shown according tothe second embodiment of the present invention. The operating principle,die size and external electrical circuit in this embodiment are the sameas those in the first embodiment with the exception that the substrate 6is using a silicon-on-insulator construction, which comprises a handlelayer 1, a device layer 2, and a silicon oxide layer 4 between handlelayer 1 and device layer 2 all connected together. The silicon oxidelayer 4 is also referred to as a “buried” silicon oxide layer and servesto be the electrical insulation between handle layer 1 and device layer2. Distinct from the first embodiment, the piezoresistive sensingelements 23 in the second embodiment are formed on device layer 2.Moreover, a silicon oxide layer 4 is formed along the sidewalls of eachpiezoresistive sensing element 23. As a result, each piezoresistivesensing element is completely wrapped around and fully insulated with alayer of silicon oxide 4 on the top, the bottom and along the sides.This dielectric isolation scheme not only reduces crosstalk andinterference among sensing elements, it also enables the pressure sensordie to operate at temperature as high as 250° C. and not limited by thePN junction insulation failure as in the first embodiment. Furthermore,type-A highly doped regions 9 are formed at the two terminals as well asat the turnaround corners of each piezoresistive sensing element 23, thepurpose of which is to increase the doping level in selective portionsof piezoresistive sensing element 23, thus forming highly conductiveregions, the function of which is the same as in the first embodiment.However, the piezoresistive sensing elements 23 in the second embodimentare single crystalline silicon resistors formed on device layer 2, thedopant type for the type-A highly doped regions 9 must therefore be thesame as the dopant type in device layer 2. In this embodiment, thethickness of device layer 2 is approximately 2 m and the thickness ofthe buried oxide layer is approximately 1 μm. However, it should benoted that the foregoing dimensions of the preferred embodiment are forillustrative purposes only. The present invention is not limited to thisembodiment and all dimensions can be tailored for a particular design.

FIG. 9 illustrates a diagrammatic view of the pressure sensor, which canbe applied to the first and the second embodiments of the pressuresensor die. Pressure sensor die 31 is installed within chamber 33, whichis an enclosure defined within a metal housing. Chamber 33 is filledwith electrical insulating fluid 37 in which pressure sensor die 31 isimmersed. In one embodiment, metal diaphragm 38 is provided to seal bothpressure sensor die 31 and electrical insulating fluid 37 within chamber33. External pressure 39 acting on metal diaphragm 38 is transmitted viaelectrical insulating fluid 37 to the pressure sensor die 31.Preferably, metal diaphragm 38 is a corrugated baffle with a lowstiffness; so almost all of external pressure 39 is transmitted topressure sensor die 31. Furthermore, the contact between pressure sensordie 31 and the chamber housing is kept to a minimum. For example,pressure sensor die 31 is only attached to the chamber housing throughone or several dots or lines of die adhesive 32. Moreover, die adhesive32 is compliant enough so that under the action of the hydrostaticpressure in electrical insulating fluid 37, pressure sensor die 31 isuniformly compressed and free to deform. Such installation furtherprevents packaging stresses, which can be caused by the deformation ofthe chamber housing due to external forces or temperature change, frombeing transmitted to pressure sensor die 31.

Preferably, within the pressure sensor die 31, the upper sealed cavityformed between substrate 6 and cap 3, and the lower sealed cavity formedbetween substrate 6 and cap 3 are both vacuum sealed, so that thepressure measured by the pressure sensor is absolute pressure referencedto vacuum. In one embodiment, metal bond pads on pressure sensor die 31are connected via bond wires 34 to metal pillars 35, which are in turnconnected to the external electrical circuit. Metal pillars 35 areelectrically insulated from one another by insulator 36.

Next the crystallographic orientations of the pressure sensor die 31 andpiezoresistive sensing elements 23 will be described. Regarding thesilicon piezoresistance effect, besides varying with stress, the siliconelectrical resistivity further varies with the dopant type (p or n),doping concentration, and crystallographic orientation since singlecrystalline silicon is anisotropic, the details for which are describedin Y. Kanda, “A Graphical Representation of the PiezoresistanceCoefficients in Silicon,” IEEE Transactions on Electron Devices, vol.ED-29, no. 1, pp. 64-70, 1982. In particular, the change in electricalresistivity and its relationship with stresses and piezoresistivecoefficients can be expressed asΔρ₁₁/ρ=π₁₁σ₁₁+π₁₂σ₂₂+π13σ₃₃+π₁₄σ₂₃+π₁₅σ₁₃+π₁₆σ₁₂  (1)where 1, 2, 3 are the three orthogonal directions in a Cartesiancoordinate system; Δρ11/ρ is the relative change in silicon resistivitywhen both the electric field and electric current are along direction 1;σ11, σ22, σ33 are the respective normal stresses along the 1, 2, 3directions; σ23, σ13, σ12 are the respective shear stresses along the2-3, 1-3, 1-2 directions; and π11, π12, π13, π14, π15, π16 are thepiezoresistive coefficients expressing the relationship betweenresistivity change and the various stresses.

Assume that the top surface of said substrate 6 of said pressure sensordie 31 is perpendicular to direction 3 on plane 1-2, and that thepiezoresistive sensing elements 23 are located on this plane. Withreference to FIGS. 2 and 5, first of all, the critical (non-highlydoped) portions of the four piezoresistive sensing elements 23 arefacing toward the upper vacuum sealed cavity. The plane 1-2 on whichthey are located is therefore a surface free from externally appliedstresses. As a result, the stress components associated with the normaldirection 3, i.e., σ13, σ23, σ33, are all zero. The critical portions ofthe piezoresistive sensing elements 23 are therefore situated in a planestress environment. Secondly, pressure sensor die 31 is installed insidechamber 33. The sensor die is uniformly compressed, and it is allowed todeform freely. Under these circumstances, the normal stresses acting onthe external surfaces of the pressure sensor die are all comparable tothe external pressure 39, and all the shear stresses are close to zero.Then further into said upper vacuum sealed cavity, on the said plane 1-2on which said piezoresistive sensing elements 23 are located, σ12 isstill close to zero, whereas σ11 and σ22 are comparable to each otherand both scale approximately linearly with the external pressure 39,labeled as P.

Summarizing all of the above, when applied to the critical (non-highlydoped) portions of piezoresistive sensing elements 23, Equation (1) canbe approximated by this simplified expressionΔρ₁₁/ρ≈(π₁₁+π₁₂ kP,  (2)where k is a constant dependent on the shape and size of the dualcavities. Since electrical resistance is proportional to resistivity,from Equation (2), the electrical resistance changes in piezoresistivesensing elements 23 scale approximately linearly with the externalpressure 39, whereas the pressure sensitivity is directly proportionalto the piezoresistive coefficients π11+π2. However, since singlecrystalline silicon is anisotropic, π11+π12 will vary according to theorientations of plane 1-2 and direction 1 as well. For example, if plane1-2 is a {110} crystallographic plane of silicon, with reference to FIG.10, when direction 1 rotates from 0° to 360°, the piezoresistivecoefficients π11+π12 will vary as shown in FIG. 10. Therefore on the{110} crystallographic plane of silicon, whether it is p-type or n-typesilicon, π11+π12 will reach a maximum along the <110> direction andreach a minimum along the <100> direction orthogonal to <110>.

As described in the first and second embodiments of the pressure sensordie 31, preferably, the two sets of piezoresistive sensing elements R1,R3 and R2, R4 can be installed along different crystallographicorientations so that when the pressure sensor die 31 is uniformlycompressed and deforms freely under the external pressure 39, thedifference in piezoresistance effect between the two sets ofpiezoresistive sensing elements R1, R3 and R2, R4, i.e., the differencein the piezoresistive coefficients π11+π12 is maximized. This way, theelectrical resistance change will be different and a voltage output willappear at the Wheatstone bridge. Preferably, the substrate 6 in thefirst embodiment and the device layer 2 in the second embodiment areformed on a {110} crystallographic plane of single crystalline silicon,and the two sets of piezoresistive sensing elements are respectivelyoriented along the orthogonal <100> and <110> crystallographicdirections. With reference to FIGS. 3 and 7, the non-highly dopedportions of piezoresistive sensing elements R1 and R3 are both orientedalong the <100> direction. Therefore their electrical resistance changesare the same. Likewise, the non-highly doped portions of piezoresistivesensing elements R2 and R4 are both oriented along the <110> direction.Therefore their electrical resistance changes are the same as well butdiffer by the maximum amount from the electrical resistance changes ofR1 and R3. Other crystallographic orientations of substrate 6 andpiezoresistive sensing elements 23 are also feasible, e.g., by referringto Y. Kanda's description.

Summarizing the dual-cavity structure of the present invention, the mainfunction of the upper sealed cavity is to provide a planar biaxialstress environment for the piezoresistive sensing elements 23, therebyfully utilizing the anisotropy of the piezoresistance effect of singlecrystalline silicon and increasing the sensitivity of the pressuresensor die. If the upper sealed cavity does not exist, thepiezoresistive sensing elements 23 will be situated in athree-dimensional hydrostatic pressure environment, and the resultingsensitivity of the pressure sensor die will become very low. The lowersealed cavity is also critical. If the lower sealed cavity does notexist, when substrate 6 and cap 3 are compressed by the external highpressure, substrate 6 will bulge toward the upper sealed cavity. Thiswill give rise to a tension on the surface region of the substrate wherethe main portion of the piezoresistive sensing elements 23 is located.This tension counteracts a portion of the bulk compression, with theresult that the normal stresses σ11 and σ22 are both less than P, andthe k in Equation (2) will be less than one. For example, an externalpressure of 200 MPa may only induce a stress of about 120 MPa to act onthe main portion of the piezoresistive sensing elements 23, resulting ina decrease in the sensitivity of the piezoresistive sensing elements.The symmetrical design of the dual cavities in the present inventionensures that under the external pressure, the substrate 6 does not bulgetoward the upper sealed cavity. As a result, only bulk compressionoccurs, and the stress induced by the external pressure is notcancelled. At the same time, the portion of the substrate 6 in betweenthe upper and lower sealed cavities becomes a main support for theentire pressure sensor die resisting the external pressure, which servesto amplify the stress. The thinner the substrate 6 is, the larger theσ11 and σ22 are in relation to P, resulting in k generally greater thanone. For example, the same pressure of 200 MPa can now induce a stressof about 220 MPa to act on the main portion of the piezoresistivesensing elements 23, and the sensitivity of the pressure sensor die isthus greatly increased.

In a conventional silicon MEMS diaphragm-type pressure sensor die, thepiezoresistive sensing elements are used to measure the maximum surfacestress on the edge of the silicon diaphragm. Since the diaphragm israther thin (typically less than 20 micrometers), the surface stressinduced by the external pressure varies substantially with positions anddepths on the diaphragm edge. Therefore a slight deviation in theposition and depth of the piezoresistive sensing elements due tofabrication processing will lead to a large deviation in the stressbeing measured. In contrast, the present invention adopts anon-diaphragm structure to convert the external pressure into bulkstress inside the substrate. The thickness of the substrate is about 200micrometers, and the variation in bulk stress at different positions anddepths of the substrate is relatively small. Therefore the effect of aslight deviation on the piezoresistive sensing elements due tofabrication processing is also relatively small.

Furthermore, in a conventional silicon MEMS diaphragm-type pressuresensor die, the pressure signal is detected by two pairs ofpiezoresistive sensing elements oriented along the same direction. Dueto the geometric constraints on placing electrical leads at thediaphragm edge, the shape and design of the two pairs of piezoresistivesensing elements often cannot be made identical. This can give rise to amismatch in the electrical resistance values or in the temperaturecoefficients of resistance between the two pairs, resulting in anincomplete cancellation of common mode errors after processing throughthe Wheatstone bridge. Although this residual error can be furthercorrected via analog or digital compensation, some pressure accuracy isinevitably sacrificed. In contrast, the present invention employs twoidentical pairs of orthogonal and symmetrical piezoresistive sensingelements to measure the bulk stress along two dimensions and then obtaintheir differential output. Hence the pressure accuracy can be higher.

Next, the fabrication process for the pressure sensor die is described.The pressure sensor die comprises three parts: a cap, a substrate and abase. The upper and lower sealed cavities do not communicate with eachother. This way the substrate does not contain any fine, fragile ormovable mechanical structure. The whole fabrication process isrelatively simple and the cost is comparatively low. Fabrication can beperformed in the order of

-   -   substrate→cap/substrate→cap/substrate/base, or    -   base→substrate/base→cap/substrate/base.        Alternatively, the substrate and the base can be separately        fabricated and then bonded one by one. The starting material for        substrate 6 can be a single crystalline silicon wafer or a        silicon-on-insulator wafer. With reference to FIGS. 11 to 15,        the first fabrication process for the first embodiment of the        pressure sensor die is described. The starting material for        substrate 6 is a single crystalline silicon wafer. This is        followed by additional process steps as described in below.

Step 1, form a layer of silicon oxide 4 on the top surface of thesubstrate silicon wafer by means of the thermal oxidation or chemicalvapor deposition method.

Step 2, using photolithography, first coat a layer of photoresist on thetop surface of the substrate silicon wafer. Then expose the top surfaceaccording to certain mask pattern. The exposed photoresist is thendissolved away with a developer, leaving the unexposed photoresist whichis subsequently hard baked. This way the exposed pattern will appear.Then using ion implantation and via energy control, the areas notcovered by photoresist are implanted with a dopant ion with sufficientenergy to penetrate the top silicon oxide layer reaching the substratesilicon wafer. Meanwhile, the ions are stopped by the photoresist in thecovered areas. This way, selective regions on the substrate siliconwafer are implanted, forming piezoresistive sensing elements 23 with adopant species of the opposite type to substrate 6. If the substrate 6is of p-type, then an n-type dopant, such as phosphorus ion, can beused. If the substrate 6 is of n-type, then a p-type dopant, such asboron ion, can be used. Lastly, the photoresist is removed. In additionto the ion implantation method, the dopant can also be selectivelyintroduced by a high temperature diffusion technique.

Step 3, using photolithography and ion implantation, form type-A highlydoped regions 9 on the top surface of the substrate silicon wafer with adopant species of the same type as piezoresistive sensing elements 23,thus forming highly conductive regions in which the electricalresistance is greatly reduced. If the substrate 6 is of p-type, then ann-type dopant, such as phosphorus ion, can be used. If the substrate 6is of n-type, then a p-type dopant, such as boron ion, can be used.

Step 4, using photolithography and ion implantation, form type-B highlydoped regions 10 on the top surface of the substrate silicon wafer witha dopant species of the same type as substrate 6, thus forming highlyconductive regions in which the electrical resistance is greatlyreduced. If the substrate 6 is of p-type, then a p-type dopant, such asboron ion, can be used. If the substrate 6 is of n-type, then an n-typedopant, such as phosphorus ion, can be used. Afterward form a siliconoxide layer 4 on the top surface of the substrate silicon wafer by meansof the thermal oxidation or chemical vapor deposition method andactivate all the implanted dopant species.

Step 5, using photolithography followed by reactive ion or plasma dryetching, or hydrofluoric acid etching, etch contact holes 8 through thesilicon oxide layer 4 on top of the highly conductive regions, therebyreaching type-A and type-B highly doped regions on substrate 6.Afterward deposit metal inside contact holes 8 and on the entire siliconwafer. Using photolithography and etching, form metal interconnectionpatterns from contact holes 8.

Step 6, bond a cap silicon wafer which has been prefabricated withrecesses to the top surface of the substrate silicon wafer in vacuum toform the vacuum sealed cavity. The bonding technique includes siliconfusion bonding, eutectic bonding, solder bonding, glass frit bonding,anodic bonding, or other thermal compression bonding methods.

Step 7, grind and thin down the bottom side of the bonded cap andsubstrate silicon wafers.

Step 8, bond a base silicon wafer 7 which has been prefabricated withrecesses to the bottom surface of the bonded cap and substrate siliconwafers in vacuum to form the vacuum sealed cavity. The bonding techniqueincludes silicon fusion bonding, eutectic bonding, solder bonding, glassfrit bonding, anodic bonding, or other thermal compression bondingmethods.

Step 9, using wafer dicing, cut the bonded cap, substrate and basesilicon wafers into completed pressure sensor dice.

Next, the second fabrication process for the first embodiment of thepressure sensor die is described with reference to FIGS. 16 to 20. Thestarting material for substrate 6 is a single crystalline silicon wafer.This is followed by additional process steps as described in below.

Step 1, fabricate recesses 5 on the top surface of a base silicon wafer7.

Step 2, bond the base silicon wafer 7 which has been prefabricated withrecesses to the bottom surface of the substrate silicon wafer in vacuumto form the vacuum sealed cavity. The bonding technique includes siliconfusion bonding, eutectic bonding, solder bonding, glass frit bonding,anodic bonding, or other thermal compression bonding methods. Afterwardgrind and thin down the top side of the bonded substrate and basesilicon wafers.

Step 3, form a layer of silicon oxide 4 on the top surface of the bondedsubstrate and base silicon wafers by means of the thermal oxidation orchemical vapor deposition method.

Step 4, using photolithography, first coat a layer of photoresist on thetop surface of the bonded substrate and base silicon wafers. Then exposethe top surface according to certain mask pattern. The exposedphotoresist is then dissolved away with a developer, leaving theunexposed photoresist which is subsequently hard baked. This way theexposed pattern will appear. Then using ion implantation and via energycontrol, the areas not covered by photoresist are implanted with adopant ion with sufficient energy to penetrate the top silicon oxidelayer reaching the substrate silicon wafer. Meanwhile, the ions arestopped by the photoresist in the covered areas. This way, selectiveregions on the bonded substrate and base silicon wafers are implanted,forming piezoresistive sensing elements 23 with a dopant species of theopposite type to substrate 6. If the substrate 6 is of p-type, then ann-type dopant, such as phosphorus ion, can be used. If the substrate 6is of n-type, then a p-type dopant, such as boron ion, can be used.Lastly, the photoresist is removed. In addition to the ion implantationmethod, the dopant can also be selectively introduced by a hightemperature diffusion technique.

Step 5, using photolithography and ion implantation, form type-A highlydoped regions 9 on the top surface of the bonded substrate and basesilicon wafers with a dopant species of the same type as piezoresistivesensing elements 23, thus forming highly conductive regions in which theelectrical resistance is greatly reduced. If the substrate 6 is ofp-type, then an n-type dopant, such as phosphorus ion, can be used. Ifthe substrate 6 is of n-type, then a p-type dopant, such as boron ion,can be used.

Step 6, using photolithography and ion implantation, form type-B highlydoped regions 10 on the top surface of the bonded substrate and basesilicon wafers with a dopant species of the same type as substrate 6,thus forming highly conductive regions in which the electricalresistance is greatly reduced. If the substrate 6 is of p-type, then ap-type dopant, such as boron ion, can be used. If the substrate 6 is ofn-type, then an n-type dopant, such as phosphorus ion, can be used.Afterward form a silicon oxide layer 4 on the top surface of the bondedsubstrate and base silicon wafers by means of the thermal oxidation orchemical vapor deposition method and activate all the implanted dopantspecies.

Step 7, using photolithography followed by reactive ion or plasma dryetching, or hydrofluoric acid etching, etch contact holes 8 through thesilicon oxide layer 4 on top of the highly conductive regions, therebyreaching type-A and type-B highly doped regions on substrate 6.Afterward deposit metal inside contact holes 8 and on the entire siliconwafer. Using photolithography and etching, form metal interconnectionpatterns from contact holes 8.

Step 8, bond a cap silicon wafer which has been prefabricated withrecesses to the top surface of the bonded substrate and base siliconwafers in vacuum to form the vacuum sealed cavity. The bonding techniqueincludes silicon fusion bonding, eutectic bonding, solder bonding, glassfrit bonding, anodic bonding, or other thermal compression bondingmethods.

Step 9, using wafer dicing, cut the bonded cap, substrate and basesilicon wafers into completed pressure sensor dice.

Next, the first fabrication process for the second embodiment of thepressure sensor die is described with reference to FIGS. 21 to 25. Thestarting material for substrate 6 is a silicon-on-insulator wafer thatcomprises a handle layer 1, device layer 2, and a buried silicon oxidelayer 4 formed between the handle layer and the device layer. This isfollowed by additional process steps as described in below.

Step 1, form a layer of silicon oxide 4 on the top surface of thesubstrate silicon wafer by means of the thermal oxidation or chemicalvapor deposition method.

Step 2, using photolithography, first coat a layer of photoresist on thetop surface of the substrate silicon wafer. Then expose the top surfaceaccording to certain mask pattern. The exposed photoresist is thendissolved away with a developer, leaving the unexposed photoresist whichis subsequently hard baked. This way the exposed pattern will appear.Then using ion implantation and via energy control, the areas notcovered by photoresist are implanted with a dopant ion with sufficientenergy to penetrate the top silicon oxide layer reaching the substratesilicon wafer. Meanwhile, the ions are stopped by the photoresist in thecovered areas. This way, selective regions on the device layer 2 of thesubstrate silicon wafer are implanted, forming type-A highly dopedregions 9 with a dopant species of the same type as device layer 2,where the electrical resistance is greatly reduced thus forming highlyconductive regions. If the device layer 2 is of p-type, then a p-typedopant, such as boron ion, can be used. If the device layer 2 is ofn-type, then an n-type dopant, such as phosphorus ion, can be used.Lastly, the photoresist is removed. In addition to the ion implantationmethod, the dopant can also be selectively introduced by a hightemperature diffusion technique.

Step 3, using photolithography on the substrate silicon wafer, followedby reactive ion or plasma dry etching, or hydrofluoric acid etching,selectively etch the top silicon oxide layer 4 to form trenches 11reaching down to device layer 2. Afterward further etch trenches 11 fromdevice layer 2 down to buried silicon oxide layer 4 using deep reactiveion etching or other dry or wet etching methods to form piezoresistivesensing elements 23.

Step 4, use the thermal oxidation or chemical vapor deposition method toform a silicon oxide layer 4 that fills trenches 11 and activate allimplanted dopant species. As a result, the piezoresistive sensingelements 23 are completely wrapped around by a layer of silicon oxideinsulation.

Step 5, using photolithography followed by reactive ion or plasma dryetching, or hydrofluoric acid etching, etch contact holes 8 through thesilicon oxide layer 4 on top of the highly conductive regions, therebyreaching type-A highly doped regions on device layer 2. Afterwarddeposit metal inside contact holes 8 and on the entire silicon wafer.Using photolithography and etching, form metal interconnection patternsfrom contact holes 8.

Step 6, bond a cap silicon wafer which has been prefabricated withrecesses to the top surface of the substrate silicon wafer in vacuum toform the vacuum sealed cavity. The bonding technique includes siliconfusion bonding, eutectic bonding, solder bonding, glass frit bonding,anodic bonding, or other thermal compression bonding methods.

Step 7, grind and thin down the bottom side of the bonded cap andsubstrate silicon wafers.

Step 8, bond a base silicon wafer 7 which has been prefabricated withrecesses to the bottom surface of the bonded cap and substrate siliconwafers in vacuum to form the vacuum sealed cavity. The bonding techniqueincludes silicon fusion bonding, eutectic bonding, solder bonding, glassfrit bonding, anodic bonding, or other thermal compression bondingmethods.

Step 9, using wafer dicing, cut the bonded cap, substrate and basesilicon wafers into completed pressure sensor dice.

Next, the second fabrication process for the second embodiment of thepressure sensor die is described with reference to FIGS. 25 to 30. Thestarting material for substrate 6 is a silicon-on-insulator wafer thatcomprises a handle layer 1, device layer 2, and a buried silicon oxidelayer 4 formed between the handle layer and the device layer. This isfollowed by additional process steps as described in below.

Step 1, fabricate recesses 5 on the top surface of a base silicon wafer7.

Step 2, grind and thin down the bottom side of the handle layer 1 of thesubstrate silicon wafer. Then bond the base silicon wafer which has beenprefabricated with recesses to the bottom surface of the substratesilicon wafer in vacuum to form the vacuum sealed cavity. The bondingtechnique includes silicon fusion bonding, eutectic bonding, solderbonding, glass frit bonding, anodic bonding, or other thermalcompression bonding methods.

Step 3, form a layer of silicon oxide 4 on the top surface of the bondedbase and substrate silicon wafers by means of the thermal oxidation orchemical vapor deposition method.

Step 4, using photolithography, first coat a layer of photoresist on thetop surface of the bonded base and substrate silicon wafers. Then exposethe top surface according to certain mask pattern. The exposedphotoresist is then dissolved away with a developer, leaving theunexposed photoresist which is subsequently hard baked. This way theexposed pattern will appear. Then using ion implantation and via energycontrol, the areas not covered by photoresist are implanted with adopant ion with sufficient energy to penetrate the top silicon oxidelayer reaching the substrate silicon wafer. Meanwhile, the ions arestopped by the photoresist in the covered areas. This way, selectiveregions on the device layer 2 of the substrate silicon wafer areimplanted, forming type-A highly doped regions 9 with a dopant speciesof the same type as device layer 2, where the electrical resistance isgreatly reduced thus forming highly conductive regions. If the devicelayer 2 is of p-type, then a p-type dopant, such as boron ion, can beused. If the device layer 2 is of n-type, then an n-type dopant, such asphosphorus ion, can be used. Lastly, the photoresist is removed. Inaddition to the ion implantation method, the dopant can also beselectively introduced by a high temperature diffusion technique.

Step 5, using photolithography on the bonded base and substrate siliconwafers, followed by reactive ion or plasma dry etching, or hydrofluoricacid etching, selectively etch the top silicon oxide layer 4 to formtrenches 11 reaching down to device layer 2. Afterward further etchtrenches 11 from device layer 2 down to buried silicon oxide layer 4using deep reactive ion etching or other dry or wet etching methods toform piezoresistive sensing elements 23.

Step 6, use the thermal oxidation or chemical vapor deposition method toform a silicon oxide layer 4 that fills trenches 11 and activate allimplanted dopant species. As a result, the piezoresistive sensingelements 23 are completely wrapped around by a layer of silicon oxideinsulation.

Step 7, using photolithography followed by reactive ion or plasma dryetching, or hydrofluoric acid etching, etch contact holes 8 through thesilicon oxide layer 4 on top of the highly conductive regions, therebyreaching type-A highly doped regions on device layer 2. Afterwarddeposit metal inside contact holes 8 and on the entire silicon wafer.Using photolithography and etching, form metal interconnection patternsfrom contact holes 8.

Step 8, bond a cap silicon wafer which has been prefabricated withrecesses to the top surface of the bonded base and substrate siliconwafers in vacuum to form the vacuum sealed cavity. The bonding techniqueincludes silicon fusion bonding, eutectic bonding, solder bonding, glassfrit bonding, anodic bonding, or other thermal compression bondingmethods.

Step 9, using wafer dicing, cut the bonded cap, substrate and basesilicon wafers into completed pressure sensor dice.

In the four fabrication processes described above, the fabricationprocess for the recesses on the cap silicon cap wafer comprisesphotolithography and etching.

The fabrication process for the recesses on the base silicon cap wafercomprises photolithography and etching.

The etching methods are selected from one or more of the followingmethods: dry etching or wet etching; the dry etching for siliconcomprises deep reactive ion etching, reactive ion etching, and gaseousxenon difluoride etching; and the dry etching for silicon oxidecomprises reactive ion etching, plasma etching, and hydrofluoric acidvapor etching.

The wet etching of silicon comprises one kind or a combination of thefollowing etchants: potassium hydroxide, tetramethylammonium hydroxideor ethylenediamine pyrocatechol.

The wet etching of silicon oxide comprises one kind or a combination ofthe following etchants: hydrofluoric acid or buffered hydrofluoric acid.

The pressure sensor die in the present invention utilizes anon-diaphragm-type novel structure. The sensor die is uniformlycompressed, and via the dual-cavity structure, the external pressure isconverted into bulk stresses inside the substrate of the sensor die.Then, the bulk stresses are converted into electrical resistance changesin the piezoresistive sensing elements 23 via the siliconpiezoresistance effect. The anisotropy of silicon piezoresistance isfurther exploited for the optimal placement of two sets ofpiezoresistive sensing elements 23 on the same crystallographic planebut along two different crystallographic orientations such that thedifference in the electrical resistance changes between the two sets ismaximized, thus enabling the measurement of pressure up to 200 MPa.Furthermore, the biaxial bulk stress induced by the external pressure ismeasured by two identical pairs of orthogonal and symmetricalpiezoresistive sensing elements, which improves accuracy. In addition,the critical portions of piezoresistive sensing elements 23 are placedinside a vacuum sealed cavity. This reduces the undesirable influencefrom the external environment and foreign materials, and increases thereliability and accuracy of the pressure sensor. Moreover, in one of thepreferred embodiments, each piezoresistive sensing element 23 iscompletely wrapped around and isolated by a layer of silicon oxideinsulator 4. Such dielectric isolation scheme enables the presentpressure sensor to operate at high temperature. Furthermore, connectingthe piezoresistive sensing elements in a Wheatstone bridge configurationis the key to reduce common-mode errors and temperature effects.Finally, manufacturing the pressure sensor die on a silicon wafer usingmicrofabrication techniques significantly reduces the manufacturing costof the pressure sensor die. As described above, a single 8-inch siliconwafer can produce thousands to over 10,000 pressure sensor dice.

Lastly, it will be appreciated by those of ordinary skill in the artthat many variations in the foregoing preferred embodiments are possiblewhile remaining within the scope of the present invention. The presentinvention should thus not be considered limited to the preferredembodiments or the specific choices of materials, configurations,dimensions, applications or ranges of parameters employed therein.

The invention claimed is:
 1. A pressure sensor die fabrication processcomprising the following steps: Step 1, grow or deposit a silicon oxidelayer on the top surface of a substrate silicon wafer; Step 2, usingphotolithography and ion implantation, dope selective regions on the topsurface of said substrate silicon wafer, thus forming a plurality ofpiezoresistive sensing elements with the opposite dopant type to saidsubstrate silicon wafer; Step 3, using photolithography and ionimplantation, highly dope selective regions on the top surface of saidsubstrate silicon wafer, thus forming highly conductive regions with theopposite dopant type to said substrate silicon wafer; Step 4, usingphotolithography and ion implantation, highly dope selective regions onthe top surface of said substrate silicon wafer, thus forming highlyconductive regions with the same dopant type as said substrate siliconwafer; afterward grow or deposit a silicon oxide layer on the topsurface of said substrate silicon wafer; and activate said implanteddopant species in said piezoresistive sensing elements, said highlyconductive regions with the opposite dopant type to said substratesilicon wafer, and said highly conductive regions with the same dopanttype as said substrate silicon wafer; Step 5, using photolithography andetching, etch contact holes through said silicon oxide layer over saidhighly conductive regions with the opposite dopant type to saidsubstrate silicon wafer, and over said highly conductive regions withthe same dopant type as said substrate silicon wafer; then use metaldeposition to form metal interconnection patterns from said contactholes; Step 6, bond a cap silicon wafer which has been prefabricatedwith recesses to the top surface of said substrate silicon wafer; Step7, grind and thin down the bottom side of said substrate silicon wafer;Step 8, bond a base silicon wafer which has been prefabricated withrecesses to the bottom surface of said substrate silicon wafer; Step 9,using wafer dicing, cut said bonded cap silicon wafer, said substratesilicon wafer, and said base silicon wafer into completed individualpressure sensor dice.
 2. The pressure sensor die fabrication processaccording to claim 1, wherein the fabrication process for said recesseson said cap silicon wafer and said base silicon wafer comprisesphotolithography and etching.
 3. The pressure sensor die fabricationprocess according to claim 1, wherein said etching step comprises onekind or a combination of dry and wet etching methods; said dry etchingmethod is selected from one or more of the following methods: deepreactive ion etching, reactive ion etching, or gaseous xenon difluorideetching for silicon; as well as reactive ion etching, plasma etching, orhydrofluoric acid vapor etching for silicon oxide.
 4. The pressuresensor die fabrication process according to claim 1, wherein saidetching step comprises a wet etching for silicon comprises one kind or acombination of the following etchants: potassium hydroxide,tetramethylammonium hydroxide, or ethylenediamine pyrocatechol.
 5. Thepressure sensor die fabrication process according to claim 3, wherein awet etching method for silicon oxide comprises one kind or a combinationof the following etchants: hydrofluoric acid or buffered hydrofluoricacid.
 6. A pressure sensor die fabrication process comprising thefollowing steps: Step 1, fabricate recesses on the top surface of a basesilicon wafer; Step 2, bond said base silicon wafer to a bottom surfaceof a substrate silicon wafer; afterward grind and thin down a top sideof said substrate silicon wafer; Step 3, grow or deposit a silicon oxidelayer on a top surface of said substrate silicon wafer; Step 4, usingphotolithography and ion implantation, dope selective regions on the topsurface of said substrate silicon wafer, thus forming a plurality ofpiezoresistive sensing elements with the opposite dopant type to saidsubstrate silicon wafer; Step 5, using photolithography and ionimplantation, highly dope selective regions on the top surface of saidsubstrate silicon wafer, thus forming highly conductive regions with theopposite dopant type to said substrate silicon wafer; Step 6, usingphotolithography and ion implantation, highly dope selective regions onthe top surface of said substrate silicon wafer, thus forming highlyconductive regions with the same dopant type as said substrate siliconwafer; afterward grow or deposit a silicon oxide layer on the topsurface of said substrate silicon wafer; and activate said implanteddopant species in said piezoresistive sensing elements, said highlyconductive regions with the opposite dopant type to said substratesilicon wafer, and said highly conductive regions with the same dopanttype as said substrate silicon wafer; Step 7, using photolithography andetching, etch contact holes through said silicon oxide layer over saidhighly conductive regions with the opposite dopant type to saidsubstrate silicon wafer, and over said highly conductive regions withthe same dopant type as said substrate silicon wafer; then use metaldeposition to form metal interconnection patterns from said contactholes; Step 8, bond a cap silicon wafer which has been prefabricatedwith recesses to the top surface of said substrate silicon wafer; Step9, using wafer dicing, cut said bonded cap silicon wafer, said substratesilicon wafer, and said base silicon wafer into completed individualpressure sensor dice.
 7. A pressure sensor die fabrication process,wherein a substrate uses a silicon-on-insulator wafer comprising ahandle layer, a device layer, and a buried silicon oxide layer formedbetween said handle layer and device layer; said fabrication processcomprising the following steps: Step 1, grow or deposit a silicon oxidelayer on a top surface of said device layer; Step 2, usingphotolithography and ion implantation, highly dope selective regions onthe top surface of said device layer, thus forming highly conductiveregions with the same dopant type as said device layer; Step 3, usingphotolithography and etching, etch trenches through said device layerreaching said buried silicon oxide layer, thus forming a plurality ofpiezoresistive sensing elements; Step 4, grow or deposit a layer ofsilicon oxide to fill said trenches, and activate implanted dopantspecies in said ion implantation step in said highly conductive regions;Step 5, using photolithography and etching, etch contact holes throughsaid silicon oxide layer over said highly conductive regions reachingsaid highly conductive regions in said device layer; then use metaldeposition to form metal interconnection patterns from said contactholes; Step 6, bond a silicon cap wafer which has been prefabricatedwith recesses to a top surface of said substrate; Step 7, grind and thindown the bottom side of said substrate; Step 8, bond a base siliconwafer which has been prefabricated with recesses to the bottom surfaceof said substrate; Step 9, using wafer dicing, cut said bonded siliconcap wafer, said substrate, and said base silicon wafer into completedindividual pressure sensor dice.
 8. A pressure sensor die fabricationprocess, wherein a substrate uses a silicon-on-insulator wafercomprising a handle layer, a device layer, and a buried silicon oxidelayer formed between said handle layer and device layer; saidfabrication process comprising the following steps: Step 1, fabricaterecesses on a top surface of a base silicon wafer; Step 2, grind andthin down a bottom side of said substrate; afterward bond said basesilicon wafer to a bottom surface of said substrate; Step 3, grow ordeposit a silicon oxide layer on a top surface of said device layer onsaid bonded substrate and base silicon wafer; Step 4, usingphotolithography and ion implantation, highly dope selective regions onthe top surface of said device layer, thus forming highly conductiveregions with the same dopant type as said device layer; Step 5, usingphotolithography and etching, etch trenches through said device layerreaching said buried silicon oxide layer, thus forming a plurality ofpiezoresistive sensing elements; Step 6, grow or deposit a layer ofsilicon oxide to fill said trenches, and activate implanted dopantspecies in said ion implantation step in said highly conductive regions;Step 7, using photolithography and etching, etch contact holes throughsaid silicon oxide layer over said highly conductive regions reachingsaid highly conductive regions in said device layer; then use metaldeposition to form metal interconnection patterns from said contactholes; Step 8, bond a cap silicon wafer which has been prefabricatedwith recesses to the top surface of said substrate; Step 9, using waferdicing, cut said bonded cap silicon wafer, said substrate, and said basesilicon wafer into completed individual pressure sensor dice.